Development of the egg is best considered in terms of a differential specification of the genome in different cell types resulting in the cellular determinations and differentiations that characterize each embryonic tissue and organ. Initially, the factors which lead to these diverse specifications are localized in the ooplasm and usually the periplasm. By utilizing the joint approaches of ultrastructure and appropriate genetic mutants found in Drosophila, we hope to gain knowledge on the structure and mode of function of these factors which appear important in cellular determination and differentiation. The research will center on: 1. the role of polar granules (germ plasm) in those cells which do not become germ cells. Two mutants will be studied: grandchildless, which possesses polar granules, but does not form germ cells; and female sterile (1) Nasrat, which may have defective polar granules and yet forms germ cells; 2. the cortical changes that occur at fertilization and specifically, the dense multivesicular bodies which form at fertilization, are localized in the cortex, and disappear at the time of determination. The fine structure of the maturation divisions will also be analyzed; 3. the mosaic structure of the cellular blastoderm, utilizing the scanning electron microscope, autoradiography, and serial thin sectioning.